Make Element Web Thread aware

[germain-gg]

Threads are now using a m.thread relation.

Before this is fully ready to ship without a labs flag, there are a couple of client considerations that we can take to ensure maximum backwards compatibility and continuity in conversations when threads will be available to the masses.

MSC3440 recommends two things for clients that do not fully support threads yet

• To treat m.thread the same way that they treat m.in_reply_to
• To change the event references from m.in_reply_to to m.thread when replying to an event that has an m.thread relation (to ensure that the message will be displayed correctly in the thread when viewed on thread's ready clients)